Don't forget that when you enable anonymous FTP that your username isn't just 'anonymous'; it is 'anonymous@yourdomain.com'.

When you use a web browser you have to pass the username as:

anonymous%40yourdomain.com

This is due to the fact that your web browser will view an '@' symbol as the delimiter between the username and the hostname, but in this particular case, the hostname is also part of the username. This is one reason why browsers make horrible FTP clients.